The invention relates to a heat insulation system which is particularly suitable for cryogenic apparatus and comprises a plurality of relatively spaced layers of a radiation-reflecting material. Heat insulation of the type concerned is described, for example, in U.S. Pat. Specifications Nos. 3,009,601 and 3,018,016 and in the article "Multilayer insulation" (Mechanical Engineering, August 1965, pages 23 to 27).
The layers generally consist of foils of a metal, for example aluminium or copper, which are spaced by, for example, glass fibers. The interstices between the foils may or may not be filled with a heat insulating filling material. Such heat insulation is particularly suitable for use in low-temperature systems such as cryostats, storage containers and pipe-lines for liquefied gas. In addition, it obviously may be used in systems of higher temperature level, such as refrigerators, deep freeze chambers, etc., and in high-temperature systems, such, as heat-accumulators in which thermal energy is stored.
Designers and manufacturers of heat insulation systems initially assumed that the insulating effect improved with increase in the number of layers (radiation shields) per unit thickness of the insulation system and hence with increase in layer density. However, measurements made on various superinsulation systems have shown that this is only partly true. When the density of the radiation shields in a given space has reached a given value, adding more shields results in a decrease in heat-insulating capacity (see for example FIG. 2 of U.S. Pat. Specification No. 3,018,016) instead of in a further increase. As an explanation of this phenomenon it has until now been assumed that this is due to heat conduction between the radiation shields owing to local point contacts. This may be a cause, but it is not the only one. Applicant has gained the insight that the impairment of the heat-insulating properties which is found when the spacing between facing surfaces of two adjacent layers is reduced, is due to the occurrence of what will be referred to as a "proximity effect." This proximity effect means that below a given minimum spacing between said surfaces, the heat transfer due to radiation between said layers increases with decreasing spacing and from a spacing slightly less than the said minimum distance is inversely proportional to the fourth power of the surface spacing. In this respect it is equally important that at the small spacings concerned, heat transfer by radiation is substantially temperature-independent (temperature dependence of the emission coefficient of the surfaces here is of secondary importance).